Acoustic absorption and spectral lines
RSI explains absorption lines as photons of exactly the right frequency lifting an electron over an Acoustic Ridge, then being re-emitted in random directions.
RSI describes absorption as emission run in reverse. When a photon, a ripple of the medium, reaches an entrained electron-knot with exactly the frequency that matches the gap between two valleys, it lifts the electron across a high-pressure Acoustic Ridge into a wider, higher-energy Acoustic Valley (n = 2, n = 3).
The absorbed energy goes into raising the electron against the proton's pull, not into speed. In the wider valley the electron moves more slowly: in hydrogen, an electron in the n = 2 valley moves at half its ground-state speed. Its energy of motion falls, the energy stored against the proton's pull rises by twice as much, and the difference is exactly the energy the photon delivered.
Only a photon with the matching frequency is absorbed. The atom soon re-emits that energy, but in a random direction, so light at that frequency is scattered out of our line of sight. Because the absorbing gas is cooler than the layers behind it, that frequency appears dark. RSI identifies this as the source of the dark absorption lines seen in the spectra of stars.
From the paper: Chapter IV, Acoustic Absorption and Spectral Lines. Read the full section
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